The present invention relates to a tilt servo system incorporated, for example, into an information read/write apparatus or the like.
The present application claims priority from Japanese Application No. 2002-215600, the disclosure of which is incorporated herein by reference.
Conventionally, an information read/write apparatus for writing or reading information on or from an information storage medium such as CDs (Compact Disc) or DVDs (Digital Versatile Disc) (hereinafter referred to as a “disc”) is provided with a tilt servo system to prevent an angle deviation (hereinafter referred to as a “tilt angle”) from occurring between the direction of emission of a light beam emitted from an objective lens incorporated into a pickup and the direction of the normal to the recording surface of a disc being illuminated with the light beam.
A prior art tilt servo system is provided at a predetermined position near an objective lens with a tilt sensor for optically detecting the angle of inclination of a disc to control the orientation of the objective lens in accordance with a sensor output from the tilt sensor, thereby preventing the occurrence of a tilt error.
The prior art tilt servo system has the property that its servo accuracy depends on various factors such as the accuracy with which the objective lens, the tilt sensor, and other associated components are attached as well as the characteristics of the tilt sensor and electronic devices for providing tilt servo control.
For these reasons, for example, upon assembling the pickup, the objective lens, the tilt sensor, and other components are assembled while the direction of the optical axis of the objective lens, the detecting direction of the tilt sensor and the like are set, in accordance with the specifications and conditions which are prescribed to realize tilt servo control with high accuracy.
However, for example, since there exist variations such as in fabrication accuracy of the objective lens, the tilt sensor, and other components, resulting in some cases in unallowable accuracy error even when the pickup is assembled in accordance with the aforementioned specifications and conditions, a final fine adjustment is made after the assembly process and before the shipment of the product.
However, since the final fine adjustment may affect other portions leading to another problem, a comprehensive and precise work for making the final fine adjustment was required to realize highly accurate tilt servo control, thereby, for example, causing the adjustment to be complicated and insufficient making it impossible to improve yields.
More specifically, by way of example, the following problems are included in the case mentioned above. To satisfy the design specifications of the prior art tilt servo system, at the designing stage before the assembly process described above, individual parts constituting the tilt servo are designed, and standard correction quantity data is prepared for appropriately making a tilt correction to the orientation of the objective lens in accordance with the sensor output of the tilt sensor.
Then, the relationship between the tilt angle θ at which a standard adjustment disc is inclined relative to the tilt sensor and the variation in the sensor output V delivered by the tilt sensor is empirically measured in advance. Then, a lookup table containing standard correction quantity data as shown by the resulting characteristic curve (1) illustrated in FIG. 14 is prepared in advance in a ROM (Read-Only memory) or the like, which is then incorporated into the tilt servo system in the assembly process.
Accordingly, when a user loads his or her desired disc into the information read/write apparatus after the shipment of the product, e.g., to write information thereon, the tilt servo system retrieves the correction quantity data for a tilt angle θs corresponding, e.g., to a sensor output Vs from the lookup table to make a tilt correction in accordance with the correction quantity data.
Although the aforementioned correction quantity data satisfies the design specifications, a fine adjustment may be further made after the aforementioned assembly process before the shipment of the product, in the case of which such a problem may be raised that the pre-standardized and fixed correction quantity data would not satisfy the condition for providing tilt servo control with high accuracy.
For example, suppose that as a result of the aforementioned fine adjustment, a tilt correction is made in accordance with the aforementioned characteristic curve (1) even though the relationship between the tilt angle θ of the objective lens and the sensor output V of the tilt sensor varies as shown by the characteristic curve (2) illustrated in FIG. 14. In this case, since a tilt angle θe that should be actually corrected cannot be determined when the sensor output V from the tilt sensor is Vs, the orientation of the objective lens would be adjusted in accordance with the correction quantity data on an erroneous tilt angle θs, thereby degrading the accuracy of the tilt correction.
For these reasons, a comprehensive and precise work for making the final fine adjustment was required to realize highly accurate tilt servo control, thereby, for example, causing the adjustment to be complicated and insufficient making it impossible to improve yields in some cases.